1. Field of the Invention
The present invention relates to a UHF-VHF combination tuner and particularly to a UHF-VHF combination tuner in which the impedance mismatching which may occur when a diode mixer is used in the UHF portion is corrected by utilizing a coil which serves as a peaking coil in the case of the VHF high channel reception.
2. Description of the Prior Art
Conventionally, such a UHF-VHF combination tuner as shown in FIG. 1 is known. In FIG. 1, reference numeral 1 designates an FET constituting a mixer stage as decribed later with respect to the invention; 2, a coil which serves as a peaking coil in the case of the VHF high channel reception; 3, a switching diode which is turned on in the case of the VHF high channel reception to thereby ground the above-mentioned coil 2 with respect to high frequencies; 4, 5, 6, 7, and 8, capacitors and inductances which constitute a UHF intermediate frequency tuning circuit; 9, a UHF mixer in the case of the UHF channel reception; 10, a DC voltage application terminal in the case of the VHF high channel reception; 11, a DC voltage application terminal in the case of the VHF low channel reception; 13, a VHF input signal input terminal; 17 a capacitor constituting a high frequency grounding circuit for the coil 2; 18, a UHF intermediate frequency input coupling capacitor having a function of DC blocking; 19, a UHF local oscillator circuit; 20, a UHF mixer tuning circuit; 21 and 22, biasing resistors; 23 and 24, resistors; and 25, 26 and 27, capacitors.
In the case as shown in FIG. 1, a fixed bias is provided to the mixer stage 1 through the resistors 21 and 22. The operation of this circuit is as follows:
[I] VHF Low Channel Reception: PA0 [II] VHF High Channel Reception: PA0 [III] UHF Channel Reception:
A DC voltage is applied to the terminal 11 shown in the drawing so as to maintain the diode 3 in its off-state. A VHF input signal is supplied to the terminal 13 and at this time the UHF portion may be considered in its inoperative state. Under this condition, the coil 2 is made to be in its nongrounded state with respect to high frequencies and the mixer stage 1 operates as a mixer for the VHF channel.
A DC voltage is applied to the terminal 10 shown in the drawing so as to maintain the diode 3 in its on-state. A VHF input signal is supplied to the terminal 13 and at this time the UHF portion may be considered in its inoperative state. Under this condition, the coil 2 is grounded with respect to high frequencies through the capacitor 17. Thus the coil 2 operates to constitute a parallel resonance circuit together with the input capacitance of the gate G1 of the mixer stage 1 to thereby improve the high band characteristic. That is, the mixer stage 1 which is provided to operate as a peaking coil is operating as a mixer for the VHF channel.
In this case, a UHF input signal is coupled to the UHF mixer 9 shown in the drawing through a not-shown arrangement. This UHF input signal is mixed in the mixer 9 with the frequency of the UHF local oscillator circuit 19 and then led to the UHF intermediate frequency tuning circuit constituted by the components 4, 5, 6, 7, and 8, through the UHF mixer tuning circuit 20. The UHF intermediate frequency passes through the UHF intermediate frequency tuning circuit and then supplied to the gate G1 of the mixer stage 1 through the capacitor 18. At this time, the mixer stage 1 operates as a UHF intermediate frequency amplifier.
Although the circuit as shown in FIG. 1 operates as described above, the matching between the intermediate frequency tuning circuit and the next stage including the mixer stage 1 is not sufficient and the noise factor is deteriorated in the case of UHF channel reception because the UHF mixer 9 employing a diode in the UHF portion is utilized. That is, when a diode is used as the UHF mixer 9, the impedance of the diode (that is the mixer 9) varies from a capacitive value to an inductive value so as to affect the above-mentioned matching characteristic, resulting in increase in loss as well as in deterioration in noise factor.